Para-xylene (PX) is a valuable chemical feedstock, which may be derived from mixtures of C8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates. The C8 aromatic fractions from these sources vary quite widely in composition but will usually comprise 10 to 32 wt % ethylbenzene (EB) with the balance, xylenes, being divided between approximately 50 wt % of meta-xylene (MX) and 25 wt % each of para-xylene and ortho-xylene (OX). Of these isomers, para-xylene is by far the most important for commercial applications.
Individual isomer products may be separated from the naturally occurring C8 aromatic mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption (e.g., the Parex™ or Eluxyl® process), or membrane separation.
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. However, since the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene, complete removal of ethylbenzene from the C8 aromatic feed by distillation is impractical. Hence an important feature of any commercial xylene isomerization process is the ability to convert ethylbenzene in the feed to useful by-products while simultaneously minimizing any conversion of xylenes to other compounds.
One xylene isomerization process that can effectively convert ethylbenzene is vapor phase isomerization, which is operated in a hydrogen-rich environment, in which a para-xylene-depleted C8 aromatics stream, mostly composed of ethylbenzene, ortho-xylene and meta-xylene, undergoes two main transformations. First, ethylbenzene is dealkylated, yielding benzene and ethylene; this reaction is immediately followed by ethylene hydrogenation to ethane in order to avoid re-alkylation reactions involving ethylene. Second, ortho-xylene and meta-xylene are isomerized to equilibrium xylenes, meaning a xylene mixture comprising close to 24 mol % para-xylene.
A typical process for para-xylene production, known as a xylenes loop, will be to described with reference to FIG. 1. A C8+ aromatics stream 100, for example, a reformate splitter bottoms stream, is provided to a xylenes column 110, where a C8 aromatics stream 112 is separated from a C9+ stream 114. The C8 aromatics stream 112 is provided to a para-xylene recovery unit 120, where para-xylene is selectively removed to yield a high purity para-xylene product 122. The para-xylene depleted effluent 124, rich in ortho-xylene, meta-xylene and ethylbenzene, is sent to a vapor phase xylenes isomerization unit 130. The isomerization effluent 136 is provided to a deheptanizer column 150, in which a C7− aromatics stream 152 is separated from a C8+ stream 154. The C7− aromatics stream 152 is fed to an extraction unit 160 and the C8+ aromatics stream 154, in which the para-xylene mol % of total xylenes is approximately 24%, is recirculated to the xylenes column 110.
The vapor phase xylenes isomerization process produces a small amount of toluene, as the result of undesirable transalkylation reactions catalyzed by the active sites of the catalyst. This toluene is typically used in a toluene disproportionation, transalkylation unit or toluene methylation process to produce more para-xylene.
The vapor phase isomerization process also produces benzene as the result of the ethylbenzene dealkylation reaction. The amount of benzene product is directly proportional to the amount of ethylbenzene in the feed. The benzene produced is typically sold as a product, requiring a sufficient purity (99.85 wt % to 99.90 wt % or higher). However, because some vapor phase isomerization catalysts produce benzene co-boilers, the C7− aromatics stream 152 (containing benzene and toluene) from the deheptanizer column 150 must be processed in an extraction unit, extractive distillation or liquid-liquid extraction, to obtain adequate purity for the benzene to be sold as a product. The extraction unit 160 removes a non-aromatic stream 162, containing the benzene co-boilers, to produce a benzene/toluene-containing stream 164, from which a benzene stream 172 is removed in a benzene column 170. The toluene-containing stream 174 is sent to a toluene column 180 to produce a toluene stream 182 and a heavier aromatics stream 184 that is recycled to the xylenes column 110.
However, because extraction is a very energy intensive process, and also because in some cases the extraction unit is operating at full capacity, it is desirable to provide a process and/or catalyst system that minimizes the amount of benzene co-boilers produced, allowing production of a high purity benzene product with the bypass of the extraction unit.